Terrorism is on the rise. A favorite target of terrorists is the public transportation systems: air, rail, bus, etc. For example, in a recent incident abroad, terrorists carried explosives onto buses and subways and detonated or attempted to detonate them. Many people were killed or wounded.
The most intense screening of passengers presently occurs at airports. Passengers are screened for the presence of metals and luggage is examined by x-ray analysis. In some installations, passengers are screened for the presence of explosives by passing air over their clothing in an attempt to extract traces of chemicals that are indicative of explosives. The air is then analyzed by a local detector. This requires the isolation and screening of individual passengers, which results in high unit screening cost and passenger traffic delays. Additionally, explosives can (and have been) hidden in shoes, which are usually independently screened by airport screeners.
The challenge is: how to prevent weapons-laden terrorists from accessing public transportation systems without substantially slowing the movement of individuals into these systems? And how to do so in a cost-effective manner.